Seedling
diseases in soybean are a minor, yet chronic problem in North Carolina.
Our mild coastal climate usually results in warmer soil temperatures earlier
in the spring than occurs in states further inland. As a result, seedling
diseases are generally of minor importance in this state. Nevertheless,
because of the wide range of planting dates and soil types encountered,
seedling diseases cause problems for some growers on a yearly basis. Seedling
diseases can cause reduced stands and limit crop growth, thus getting
the crop off to a slow start. These factors may lower yields and make
management of other pest problems more difficult.

Seedling
diseases tend to be more severe in poorly drained soils. They can, however,
present problems in any soil type, especially when high rainfall or cold
weather follow planting. Any factor that delays germination and seedling
emergence such as poor seed quality, inadequate seedbed preparation, compaction,
planting too deep, nematode infestations, and high rates of some herbicides
can contribute to the incidence and severity of seedling diseases.

Most soybean
fields receive one or more herbicides, and most of the available soybean
herbicides have the potential to injure soybeans. Some can generate visual
symptoms that are similar to disease symptoms. Soybean tends to be more
seriously injured by broadleaf herbicides than grass herbicides. Pre-emergence
herbicides whose label suggests planting soybean below a certain depth
probably have the greatest potential to injure soybeans if the herbicide
gets into the soybean root zone.

Distinguishing
between herbicide injury and seedling diseases is frequently difficult.
The pattern of where symptoms do and do not appear may give the best indication
of whether it is a disease problem or herbicide injury. Symptoms of seedling
diseases usually occur in irregular patterns which may correspond to changes
in soil type. A disease is more likely to be severe on one plant and show
no symptoms on adjacent plants than is herbicide injury. The pattern of
herbicide injury symptoms typically appear to be equipment related, although
they may be modified by differences in soil type. Weed control is often
excellent in the affected areas, and seldom will one plant be seriously
affected and the adjacent plants appear unaffected. Finding an area in
the field the sprayer may have missed, or overlapped in a previous pass,
can be very useful in diagnosing herbicide injury.

Soybean seedling
diseases are caused by fungi that reside in the seed or the soil. Pythium
spp., Phytophthora sojae, and Rhizoctonia solani are the
soil fungi most commonly associated with seedling diseases. Pythium
and Rhizoctonia are found in all agricultural soils. These fungi
will commonly cause a root rot and either lesions on the stem or soft
watery stem tissue. See the figure at the top of this note.

Sore shin (a red-to-brown lesion at or above the soil line), damping off
and root rot can be caused by Rhizoctonia solani. Plants frequently
recover from the root rot phase if optimal conditions for soybean growth
occur soon after emergence. This fungus affects many plant species and
is found in all soils. Other common soil inhabitants are various species
of Pythium. Pythium rot can occur at any stage of plant development,
but is primarily a seedling disease favored by high moisture. A variety
of symptoms are associated with this disease: a seed rot, a wet rot (soft
watery stem tissue), baldhead (retarded development of the growing point),
swelling of the stem below the cotyledons (which can be confused with
herbicide injury), a root rot and/or wilt and death of seedlings. Diseases
caused by Pythium spp. and Rhizoctonia may be suppressed,
but not eliminated by rotation with grass crops such as corn or sorghum.
Although rotation is not an especially effective tactic for managing these
diseases, the reduction in other soybean pests which occurs with rotation
generally results in a lower incidence of these seedling diseases. Seed-treatment
fungicides containing thiram or PCNB are reasonably effective against
Rhizoctonia solani, whereas seed-treatments containing mefanoxam
(Ridomil Gold, Apron) are necessary if Pythium is the problem.

The soil
fungus responsible for Phytophthora rot of soybean, Phytophthora sojae,
is similar to Pythium spp. It is most commonly a problem in heavy,
poorly drained soils or in low spots in the field. Diseases caused by
this organism range from seed rot to stem and root rot, which may result
in plant death. Rotation with non-leguminous crops (two or more years
may be required), resistant or tolerant varieties and fungicide (mefanoxam)
treatments can be used to manage this pest.

Several fungi
that cause seed decay and seedling diseases are seedborne. Phomopsis seed
decay, frogeye leaf spot (SOY003),
anthracnose, purple seed stain, and downy mildew can cause seed rots,
reduce emergence and prevent adequate stand establishment. These problems
are usually more severe in cool wet soils. The best tactic for avoiding
these diseases is the use of high quality disease-free seed.

The most
common of these seedborne diseases is phomopsis seed decay. Seed infected
with Phomopsis spp. typically have greatly reduced seed germination
in a cold test compared to a standard germination test. This seed decay,
like most other seedborne diseases, is a result of delayed soybean harvest
and humid conditions during seed development. Pod and stem blight caused
by Phomopsis spp. is present in nearly all fields at soybean maturity.
If soybean harvest is late and warm humid conditions persist, infection
of the seed will occur. Seed from these plants may not germinate and may
produce seedlings lower in vigor. The other seedborne fungi mentioned
cause similar problems, but are less common. Late-season scouting of soybean
fields should include identification of foliar, pod, and stem diseases
if soybeans are grown for seed. Seed should not be saved if anthracnose
or frogeye leaf spot are present in the field. Your County Agent can forward
plant samples to the Plant Disease and Insect Clinic if you are uncertain
as to exactly which diseases are present. Seed saved for planting next
year should be tested for germination. Send seed lots to: NCDA-Seed Laboratory,
P.O. Box 27647, Raleigh, NC 27611. If seed of dubious quality must be
planted, the use of a fungicide-seed treatment may prove beneficial. Seed
treatments containing carboxin and/or captan are most effective against
these fungi with the exception of downey mildew, which can be controlled
with metalaxyl.

Soybean can
yield well over a relatively wide range of plant population densities.
Unless populations are so high that lodging becomes a problem or so low
that the canopy does not close, yields will not be affected markedly.
Target population densities are 6-8 plants per foot of row with 36-inch
row spacing and 2 per foot of row in 7-inch rows for May-planted soybean.
A higher population is desirable in June with 9-11 plants per foot of
row and 2.5-3 plants per foot of row with 36- and 7-inch rows, respectively.

If the remaining
plants are healthy, and fairly uniformly distributed, stands of half the
levels in the previous paragraph are probably adequate. The added productivity
of the replanted crop would have to be enough to cover the additional
twenty to thirty dollars per acre cost of replanting. The later planting
date lessens the likelihood that this will happen. The greatest uncertainty
comes in guessing the current and future health of the remaining plants.
When evaluating an uneven stand of very young soybean plants, it is difficult
not to view the remaining stand pessimistically. Probably more fields
are replanted which did not need to be than fields not replanted which
should have been.

The herbicides
used on the field must be considered when a decision to replant is made.
Do not apply more pre-emergence herbicide with the second planting. Scout
the field periodically to determine if a post-emergence treatment is necessary.
Tillage can be used to dilute the concentration of the broadcast herbicide
if it is labeled for preplant incorporation and is suspected of being
part of the problem. If the broadcast herbicide is not labelled for preplant
incorporation (e.g., linuron and linuron tank mixes), or does not appear
to have been leached deep enough into the seedbed to be causing injury
to the first crop, additional tillage should not be performed before replanting.
If a soil-applied herbicide was banded over the original planting, and
the decision is made to replant between the original rows (and thus away
from the banded herbicide) a banded herbicide could also be used over
the replanted rows as well, provided the sum of the two banded applications
do not exceed the labelled per-acre rates for that herbicide.

Published
by North Carolina Cooperative Extension Service

Distributed
in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment
and program opportunities are offered to all people regardless of race,
color, national origin, sex, age, or disability. North Carolina State
University at Raleigh, North Carolina A&T State University, U.S. Department
of Agriculture, and local governments cooperating.

Recommendations
of specific chemicals are based upon information on the manufacturer's
label and performance in a limited number of trials. Because environmental
conditions and methods of application by growers may vary widely, performance
of the chemical will not always conform to the safety and pest control
standards indicated by experimental data.

Recommendations
for the use of chemicals are included in this publication as a convenience
to the reader. The use of brand names and any mention or listing of
commercial products or services in this publication does not imply endorsement
by the North Carolina Cooperative Extension Service nor discrimination
against similar products or services not mentioned. Individuals who
use chemicals are responsible for ensuring that the intended use complies
with current regulations and conforms to the product label. Be sure
to obtain current information about usage and examine a current product
label before applying any chemical. For assistance, contact your county
North Carolina Cooperative Extension Service agent.

Last
update to information: May 2000
Last checked by author: May 2000Web page last updated on May 2000 by A.V.
Lemay.